1. Field of the Invention
The invention relates to a confocal optical microscope and length measuring device using this microscope.
2. Description of Related Art
Conventionally a confocal optical microscope is used, applying its characteristic of a depth of focus which is extremely short, for viewing shapes such as cross-sectional shapes or the like, fine objects in biology, semiconductor technology or the like, and for purposes of various measurements in a suitable manner.
In addition, conventionally, for example, for measuring the length of a line width of a resist pattern formed on a semiconductor wafer (width of a resist line) or a pattern distance (distance between adjacent resist lines) or for length measurement of a photomask pattern for exposure, a length measuring device is used which has an optical microscope. The semiconductor wafer is hereinafter called only a "wafer".
Since a resist layer thickness according to a refinement of the pattern has recently tended to become smaller as well, there is a need for a length measuring device using a microscope with a shallow focal depth. With respect to the facts described above, a length measuring device using a confocal optical microscope has been developed which has the advantage that its focal depth is low to the greatest degree.
FIG. 6 schematically illustrates one important configuration of a conventional confocal optical microscope which is installed in a conventional length measuring device. In the representation, reference numeral 1 designates an illumination optics system, 2 a first polarizer consisting of a polarization plate, 3 a polarizing beam splitter, 4 a aperture plate with a pinhole, 5 a first objective, 6 an imaging lens, 7 a second objective, 8 a second polarizer consisting of an analyzer, 9 a second imaging lens, 10 a picture recording means consisting of a camera and the like, and 11 a quarter wave plate.
In a confocal optical microscope with this type of configuration, radiation light from illumination optics system 1 is converted by means of first polarizer 2 into a linear polarization. Subsequently, by means of polarizing beam splitter 3 it is reflected and locally irradiates one surface of aperture plate 4 which has the pinhole. An area which is locally irradiated in this process corresponds essentially to a field of view of the picture recording means 10. The term "field of view of picture recording means 10" is defined as an area on the aperture plate in which the picture recording means can gather light.
In the light for local irradiation, a light flux which has travelled through the pinhole of aperture plate 4 is converted into parallel light or essentially parallel light by means of first imaging lens 6 and is incident on first objective 5. Between this imaging lens 6 and objective 5 is quarter wave plate 11. The linearly polarized light which is aligned from first imaging lens 6 towards objective 5 is converted by one pass through this quarter wave plate 11 from linear polarization into circular polarization.
The light incident on first objective 5 is concentrated on one surface of sample S to be measured. In this way, a point of sample S to be measured is irradiated.
Reflected light from sample S to be measured travels again through first objective 5, quarter wave plate 11 and first imaging lens 6 and is again imaged in the pinhole of aperture plate 4. The reflected light which is directed from first objective 5 to first imaging lens 6 is converted by passing once again through quarter wave plate 11 from circular polarization into linear polarization which has a polarization direction turned 90.degree. relative to the illumination light.
The light reflected by polarizing beam splitter 3, as the result of passing twice through quarter wave plate 11 before and after irradiation of sample S, travels through aperture plate 4 in a state of linear polarization. Since, however, in doing so the polarization directions are different, the reflected light which has travelled through the pinhole of aperture plate 4 after one passage through second objective 7 passes through second polarizer 8 and second imaging lens 9 and is gathered up by picture recording means 10 without being reflected by polarizing beam splitter 3.
Reflected light which has travelled through the pinhole of aperture plate 4 travels after one passage through second objective 7, polarizing beam splitter 3, second polarizer 8 and second imaging lens 9 and is gathered up by picture recording means 10.
In FIG. 6, a broken line designates an optical path of all illumination light which reaches as far as aperture plate 4 from the illumination optics system 1, and a solid line designates an optical path in which the light which has travelled through a pinhole of aperture plate 4 is concentrated on sample S and reflected, and then reaches picture recording means 10.
By means of the arrangements of first polarizer 2 (polarizing plate) described above in the optical path between illumination optics system 1 and the polarizing beam splitter 3, second polarizer 8 (analyzer) in the optical path between polarizing beam splitter 3 and the picture recording means 10 as well as quarter wave plate 11 in the optical path between the first objective 5 and the first imaging lens 6, the reflected light from sample S is differentiated from the scattered light reflected from the surface of aperture plate 4; the scattered light is for the most part not gathered by picture recording means 10. In this way, a reduction of the contrast, which is caused by gathering scattered light from the surface of the aperture plate 4, is prevented and a relatively good image can be observed.
A confocal optical microscope with the configuration described above is disclosed, for example, in the Japanese patent publication HEI 1-503493. In this publication, a scanning disk (rotating Nipkow disk) provided with pinholes is used as the aperture plate, by means of which measurement of an entire image of the sample in real time is enabled.
In a length measuring device which has a confocal optical microscope with the configuration described above, a length of material components of the aforementioned sample to be measured which was described above (for example, the magnitude of the width of a resist line) and a length between the material components described above (for example, a distance between resist lines) are measured based on an enlarged image which is picked up by the confocal optical microscope. To take an exact measurement, therefore, it is necessary that the contrast of the aforementioned enlarged image has a certain level. In the case in which the contrast is low, it becomes difficult to recognize and determine positions of starts and ends (for example, ends of the resist line) of the material component of the sample to be measured, the length of which or the lengths between which must be measured. Consequently, the reliability of measurement results decreases.
For the confocal optical microscope shown in FIG. 6, by means of the configurations of first polarizer 2 (polarizing plate) in the optical path between illumination optics system 1 and polarizing beam splitter 3, the second polarizer 8 (analyzer) in the optical path between the polarizing beam splitter 3 and picture recording means 10, as well as the quarter wave plate 11 in the optical path between the first objective 5 and first imaging lens 6, the reflected light from sample S to be measured is distinguished from the scattered light reflected from the surface of aperture plate 4; the scattered light is for the most part not gathered by the picture recording means 10. In this way a reduction of the contrast is prevented which is caused by gathering scattered light from the surface of aperture plate 4.
The conventional confocal optical microscope and conventional length measuring device with a confocal optical microscope, however, have the following disadvantages:
(1) In a confocal optical microscope with the configuration shown in FIG. 6, the light flux of the light which travels through a pinhole of aperture plate 4, and which is incident on first objective 5 from the imaging lens 6, is converted into an essentially parallel light flux.
Consequently, as shown in FIG. 5, the reflected light 91 which is part of the light which has travelled through the pinhole of aperture plate 4 and which was reflected from the surface of quarter wave plate 11, as well as reflected light 92 which has been reflected from the rear side of quarter wave plate 11, after passage through first imaging lens 6 are concentrated again into the pinhole in the locally irradiated area of aperture plate 4.
The reflected light which was concentrated in the locally irradiated area which essentially corresponds to the field of view of picture recording means 10, including especially reflected light 92 which was reflected from the rear side of quarter wave plate 11 as the result of the polarization direction turned 90.degree. relative to the illumination light is finally, for the most part, gathered by picture recording means 10, and furthermore, as the result of passing through quarter wave plate 11 essentially twice, has the same polarization characteristic as the light reflected from sample S. The light is finally gathered by picture recording means 10 and reduces the image contrast as a stray beam, which has been reflected from locations other than from sample S.
This reduction of contrast which is caused by reflected light from the surface and/or the rear side of the quarter wave plate 11 reduces measurement accuracy and the reproducibility of the length measuring device and thus adversely affects the reliability of measurement results.
(2) Upon repeated uses of the confocal optical microscope, an adhesive for joining the lenses and lens systems which comprise the optical systems of the microscope is degraded over time, and light transmission in these optical systems decreases after a relatively short time. With the decrease of light transmission, the image to be observed becomes darker overall and difficult to recognize.
The measurement accuracy and reproducibility of the length measuring device decrease and the reliability of the measurement results is adversely affected because 1) as the result of a small amount of light for picture recording means 10, a sufficient SN (signal to noise) ratio is not obtained and 2) as the result of a fluctuation of measurement conditions which occurs over time (a reduction in the amount of light over time), measurement errors arise.